Here's the way I've been able to do it; it works for individual rooms, regardless of the shape. To get total square footage of the entire structure, I had to do that myself by adding up each and every room/space (I did it on a spreadsheet); okay, here's the way:
To get square footage of rooms in DreamPlan Plus by NCH Software:
get in a 2D view (either 2D Blueprint or 2D Rendered) - do this by clicking on the icon on the right side of the screen or by clicking on the mini map in bottom-right, then click on one of the 2D options from the pop-up window).
select the "Tools" tab on the top-left
select "Text Label" on the row just below that
in the left-side vertical bar section, it should be titled: "Text Label Properties", under "Label Type:", select "Room Label"
hover the mouse over one of your rooms and it should now be highlighted (in blue); if you have multiple rooms connected, the highlight should change to only one at a time, the one you are currently hovering the mouse cursor over
to get the square footage of a room, click the mouse when that room is highlighted
a text box will appear in the center of the room that says: "<Room Name>" and then below that a number and then "sq. ft" (or "sq. m." if using metric) --- Also, you can look back to the vertical bar section on the left side of the window and it will display "Text Label:" with a little box to edit the name if you wish. It also lists the square footage here.
If you click out of that room and the software automatically goes back to a different tab, (if you're trying to get multiple rooms like I was painstakingly trying to do) then just re-click the "Tools" tab and you should be able to select a room of your choice to get the square footage again. Also, you may have to re-click the "Room Label" selection if that gets changed. If you want a quick way to label each room without having to re-enter the tools menu, then before you select the next room, go back and click "Text Label" or "Room Label" and then you can select the next room.
P.S. You can also get the length of the walls by selecting "Wall Label" instead of "Room Label" in step 4 above, then clicking on a wall when the hand icon appears